The present invention relates to method and device and for economically storing auxiliary correction data for positioning a Global Navigation Satellite System (GNSS) receiver, and more particularly, to method and device for economically storing Spaced Based Augmentation System (SBAS) correction data in a GNSS receiver.
Recently, the GNSS is well known in the art and are commonly used to determine the geodetic latitude and longitude coordinates of mobile vehicles employing such devices. The current GNSS includes Global Position System (GPS), Galileo, GLONASS and other satellite positioning technologies. For simplicity, a GPS device will be discussed herein as an example of a GNSS, wherein the terms “GNSS” and “GPS” may be used interchangeably.
The GPS has become one of the more popular navigation systems in the world, and as such is currently applied in a wide variety of fields. The GPS Operational Constellation consists of a plurality of satellites that orbit the earth and constantly broadcast its location information from space. GPS receivers can detect these signals and convert these signals into position, velocity, and time markers. Generally, at least three satellites are required for a GPS receiver to compute a two dimensional position (Latitude, Longitude) of the GPS receiver, with more satellites required for computing a three dimensional position (Latitude, Longitude, Height). A GPS receiver can determine its position by determining its distance from the GPS satellites according to the received signals and can calculate its location within 10 meters of accuracy.
However impressive, the current GPS system still possesses some limitations due to various potential errors. For example, the ionospheric and tropospheric disturbances can cause a delay of GPS satellite signals and thereby cause signal carriers and codes received by the GPS receiver to become distorted. Since these ionospheric and tropospheric disturbances are unpredictable and can change significantly from location to location and time to time, these errors are difficult to correct with current GPS receiver technologies. In order to solve this problem, SBAS have been developed to further account for the above-described errors and better improve the accuracy, availability and integrity of the GPS. The current SBAS includes Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), MSAS, etc. For simplicity, a WAAS will be discussed herein as an example of a SBAS, wherein the terms “SBAS” and “WAAS” may be used interchangeably.
WAAS was developed to improve the accuracy, availability and integrity of GPS. The WAAS is based on a network consisting of a multitude of wide area ground reference stations (WRS). Each WRS can receive and analyze signals from GPS satellites within its coverage area and determine relative errors in the signals. A wide area master station (WMS) then collects the error data from each WRS and computes correction information for each GPS satellite based on specific correction algorithms. Correction messages are then uplinked to WAAS satellites, which broadcast these correction messages on the same frequency to the GPS receivers within the coverage area of the WAAS satellites. GPS receivers can then utilize the WAAS correction information to correct for GPS satellite signals errors caused by timing and distance inaccuracies, or errors caused by ionospheric disturbances.
Typically, the WAAS satellites broadcast three major types of correction data: fast correction data, long term satellite error correction data and ionospheric delay correction data. For the fast correction data and long term satellite error correction data (called fast/long term correction data hereinafter), correction data for maximum 51 satellites can provided by a WAAS satellite. Therefore, some GPS receivers prepare 51 memory spaces for storing those fast/long term correction data. The method is easy but very space consuming. One much space efficient method is only to collect and store correction data for those satellites that are currently in tracking channels. The maximum number of memory spaces required by a GPS receiver depends on the amount of tracking channels it can afford. For example, a 14-channel GPS receiver can simultaneously track up to fourteen GPS satellites and only store the fast/long term correction data corresponding to these fourteen GPS satellites from a WAAS system. The GPS receiver can then calculate and correct the positional information that is provided from the GPS satellites according to the stored fast/long-term correction data. However, the current method for storing fast/long term correction data corresponding to the tracked satellites forces the GPS receivers to wait for collecting WAAS correction data. One dilemma that may occur is when a new satellite is introduced into a tracking channel of a GPS receiver. Because the GPS receiver would not contain the fast/long term correction data for the new satellite, it must spend an extra period of time to wait for and receive new fast/long term correction data for the new GPS satellite from the WAAS satellite. The resulting delay inhibits WAAS functionality and prevents continuous and smooth operation of the GPS receiver.
Additionally, ionospheric delay correction data is broadcasted for selected ionospheric grid points (IGP) spaced at approximately 5 degree intervals in latitude and longitude. Please refer to FIG. 1. FIG. 1 is global map that illustrates the global ionospheric grid point (IGP) correction locations according to the related art. As shown in FIG. 1, there are 1808 IGPs defined around the surface of the earth horizontally distributed into nine areas (bands). One method of storing ionospheric delay correction data is to store all of the available correction data of the 1808 IGPs within the GPS receiver. Since most GPS receivers have limited memory and processing capabilities, they cannot quickly and efficiently store and process data from all the 1808 ionospheric grid points. Even if a GPS receiver does have sufficient memory capacity to store all the ionospheric grid point correction data, it would be inefficient to do so because only the localized ionospheric grid points near the GPS receiver are relevant to the ionospheric conditions at that location.